This invention relates to rotogravure printing, i.e., printing from recessed wells that hold ink on a rotating printing plate or surface. A basic problem in gravure is to photographically translate a pattern of color tones, highlights and shadows into a pattern of recessed wells on the printing plates. These recessed wells receive the various colored inks which are then printed on paper. Over the course of development of rotogravure which extends well over fifty years, a number of methods or processes have been developed for producing gravure plates. In what is commonly termed "conventional gravure" a separate negative is made for each of the four colors used in full-color printing by photographing the copy through a different filter for each color. The negative plates receive extensive retouching and layout so that in the final reproduction there is a near correct relationship between the various elements of the copy. The negative plates are then utilized for making a continuous tone positive, again, one for each color. The positives are carefully inspected and additional retouching is performed. The corrected film positives are then used to expose a carbon tissue/resist used to etch the printing surface.
The carbon tissue is a pigmented gelatin emulsion which is sensitive to light. In the conventional gravure process, it is placed in a vacuum frame in contact with a 150-line conventional gravure screen and exposed to light. The light passing through the screen hardens the tissue, causing a screen pattern to form on the carbon tissue. Next, each film positive is placed in a vacuum frame with the carbon tissue and light is passed through the positives to transfer their image to the carbon tissue.
The carbon tissue thus prepared is laid on to the copper cylinders and a standard chemical etching process is performed whereby the exposed portions of the carbon tissue form a mask or resist. The resulting etched cylinder comprises a plurality of round or square wells which vary in depth.
In a modified version often referred to as the Dultgen Halftone Process, a printing cylinder is produced having squares or dots etched therein which vary in depth as well as in width. The Dultgen process is disclosed in U.S. Pat. No. 2,096,794. In the Dultgen process a soft focus negative is produced. From this soft focus negative a continuous tone positive is produced and used to expose the carbon tissue. The tissue is also exposed a second time, in several embodiments, by a halftone plate positive. After the image of both positives is transferred to the carbon tissue upon which a grain screen has been exposed, etching is accomplished in the conventional manner.
A study of the Dultgen patent discloses that in one embodiment a gravure printing plate is produced without exposing a grain screen on the carbon tissue. In this embodiment a halftone screened negative and a soft focus negative are used, a halftone screen being an alternating light and dark checkerboard arrangement. It is proposed that these two negatives be superimposed in register to produce a combined positive or alternately be used to produce separate positives which are used to twice expose the carbon tissue. As will be explained, the single positive variation using a halftone screen and glass positives does not produce satisfactory plates. Principally, this was due to the use of glass positives, the soft focus and the use of a halftone plate or screen.
The two-positive technique, however, is satisfactory especially when a lateral pillow cushion screen is used and has become a standard technique in the industry commonly referred to as the "hard dot process". In the hard dot process one lateral screened positive and one continuous tone positive are separately exposed on the carbon tissue resist prior to etching the copper cylinder. In using the hard dot process it is still necessary to perform the normal hand correcting of the continuous tone negative as well as highlighting, outlining, retouching of the negatives and the positives, and proofing the positives, as required for most gravure techniques.
It is accordingly an object of the present invention to provide an improved method for producing gravure etchings.
It is another object of the present invention to produce a gravure method for producing etchings which utilizes a single positive for producing the carbon tissue resist.
It is another object of the invention to provide a method of producing gravure printing surfaces without the need for substantial manual correcting by skilled gravure artists.
It is a further object of the present invention to provide a method of producing gravure printing which is low in cost and significantly faster than conventional techniques.
It is a further object of the present invention to provide a gravure etching method which eliminates a substantial portion of the negative and positive correcting and layout.
By way of comparison, in the current hard dot process, two positives are produced and separately printed upon the carbon tissue to produce the plates. The present method utilizes one positive for exposing the carbon tissue and thus eliminates the problem of maintaining register stability during exposure of the tissue. A 50% savings in time and clarity is often realized, as well as better color register.
Whether monotone or color etchings are being produced, the present method reduces the cost of producing the positives for etching the plates with a significant increase in quality. Truer color correction is inherently obtained because of the use of orthochromatic film. Corrections, such as are necessary, are all done photographically in a manner similar to the techniques currently used in offset printing. A significant reduction in the number of intermediate films used in conventional gravure techniques is also obtained.
Other objects and advantages of the invention will be apparent from the concluding portion of the specification.